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Rheological behavior of wood in stress relaxation under compression
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Mechanics. Univ Boras, Fac Text Engn & Business, S-50190 Boras, Sweden.
Univ Ljubljana, Fac Civil & Geodet Engn, Ljubljana 1000, Slovenia.
Univ Boras, Fac Text Engn & Business, S-50190 Boras, Sweden.
2018 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 52, no 3, p. 793-808Article in journal (Refereed) Published
Abstract [en]

Rheological behavior of wood under uniaxial compression along and perpendicular to the grain in constant environment was examined. Tests with constant deformation rate until failure and stress relaxation tests with constant deformation applied stepwise were carried out. The experimental results of stress relaxation showed nonlinear material behavior over time that got more prominent under high deformation levels. Considerable amount of stress relaxed during applying the deformation. Wood experienced greater stress relaxation along the grain than perpendicular to it. Three rheological models for orthotropic material were calibrated to the experimentally determined stress-time curves in longitudinal and transverse directions simultaneously. Small deformation levels assuming linear strains were accounted for in the models. Required elastic material parameters were determined from the tests with constant deformation rate. A model including the highest number of viscoelastic material parameters was the most successful in predicting stress relaxation of wood under stepwise deformation. Modeling indicated that wood behavior was very close to linear viscoelastic in relaxation under small deformation. The obtained material parameters made the model suitable for predicting rheological behavior of wood comprehensively, under sustained deformation or load in constant conditions.

Place, publisher, year, edition, pages
SPRINGER , 2018. Vol. 52, no 3, p. 793-808
National Category
Composite Science and Engineering
Identifiers
URN: urn:nbn:se:uu:diva-352579DOI: 10.1007/s00226-018-0993-2ISI: 000430010200011OAI: oai:DiVA.org:uu-352579DiVA, id: diva2:1237094
Available from: 2018-08-07 Created: 2018-08-07 Last updated: 2019-01-28Bibliographically approved
In thesis
1. Moisture-Induced Strains and Stresses in Wood
Open this publication in new window or tab >>Moisture-Induced Strains and Stresses in Wood
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

To design safe, reliable and durable timber structures subjected to varying natural outdoor or indoor climates, understanding the long-term behavior of wood when mechanically loaded or restrained to deform is crucial. The present thesis focuses on the numerical modeling of the long-term mechanical behavior of wood. The numerical analysis is divided in the moisture transport and the mechanical analyses. In the moisture analysis, the multi-Fickian moisture transport model is used to determine spatial and temporal moisture content fields over the analyzed domain due to changing relative humidity (RH) of the ambient air. The obtained moisture contents are taken into the mechanical analysis where a new mechanical model is applied for predicting rheological response of wood in three orthotropic directions simultaneously. Experimental results of different authors are used to support numerous numerical analyses performed for various wood species, deformation and loading modes in constant or changing RH conditions. The performed analyses show that the new mechanical model adequately predicts the viscoelastic behavior of hardwood and softwood species in two orthotropic directions simultaneously under a sustained load or deformation. A significant influence of grain orientation in relation to the applied mechanical load on the viscoelastic creep behavior of wood is observed. The mechanical model is also able to predict accurately the rheological behavior of hardwood subjected to a sustained compressive mechanical load and changing moisture content. Applying the moisture and the mechanical models to the glued-laminated timber specimens during wetting and drying shows good agreement with the experimental results. The magnitudes of moisture-induced stresses perpendicular to the grain indicate a possibility of crack initiation during drying. The influence of characteristic material parameters required in the models on the mechanical state of the analyzed specimens is also determined. A quantification of the viscoelastic and the mechanosorptive material parameters required in the mechanical model is the additional outcome of the performed numerical analyses. The mechanical model presented in this thesis in combination with the multi-Fickian moisture transport model enables a full two- or three-dimensional long-term mechanical analysis of timber members exposed to natural climate with RH variations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 51
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1772
Keywords
mechanosorption, multi-Fickian moisture transport model, orthotropy, rheology, three-dimensional mechanical model, viscoelasticity, wood
National Category
Applied Mechanics
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-375148 (URN)978-91-513-0569-1 (ISBN)
Public defence
2019-03-22, Häggsalen Å10132, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2019-02-27 Created: 2019-01-28 Last updated: 2019-03-18

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